In the aftermath of a heart attack, the identification and assessment of non-viable (necrotic) tissues is necessary for effective development of intervention strategies and treatment plans for certain types of heart disease. Those tissues which are healthy or capable of recovery through coronary bypass, stent placement, etc., should be distinguished from those which are non-viable, or irreversibly damaged. In this way, predictions may be made as to which patients might benefit from revascularization so as to increase their cardiac function and survival rate.
Physicians rely on several non-invasive indicators for determining viability of myocardial tissue. The morphology of the myocardium, specifically its thinning, is evidence of necrotic tissue. In addition, abnormal motion, such as passive movement of a region or in extreme cases no movement at all can indicate myocardial damage. However, although morphology and functional changes are indicative of tissue abnormality, they are not sufficiently sensitive to differentiate abnormal from non-viable (dead) tissue.
Contrast enhanced imaging techniques may also be used to help identify non-viable regions. Positron-emission tomography (PET) and single-photon-emission tomography SPECT are capable of indicating viability information through differential signal intensity. However, these modalities are of limited utility as their resolution is quite low and are not generally available in the case of PET.
Recently a new contrast enhanced imaging technique, Delayed Enhancement Magnetic Resonance (DEMR) has been shown to enable direct visualization of non-viable myocardium. DEMR imaging is a technique whereby non-viable myocardial tissue appears with increased signal intensity. DEMR is typically performed using a standard inversion recovery MRI acquisition sequence 20-30 minutes after administration of paramagnetic contrast agent (e.g., Gd-DTPA). Furthermore, DEMR has sufficient spatial resolution to accurately distinguish viable (normal or ischemic) from non-viable myocardium within the left ventricular wall. Radiologists typically acquire these images in conjunction with other functional modalities (e.g., MR Cine), and use domain knowledge and experience to isolate the non-viable tissues.
Even with the information provided above, determination of tissue viability can be challenging. First, DEMR is prone to false negatives. That is, a region which is non-viable may not have increased signal intensity. Second, assigning viability status based on morphology and differential morphology (thickening or wall motion) requires a certain amount of experience and intuition. And, the indicators describing abnormal wall motion may be conflated. In other words, is a particular region of the heart moving independently or is it being pulled along by a neighboring region? In short, it may take an expert to label a region as non-viable.
Despite the difficulty in its identification, there remains a great deal of interest in locating and quantifying non-viable tissues because it has been shown that the extent of infarction is closely related to long-term improvement in cardiac function after coronary revascularization. However, DEMR is a recent technique not yet approved by the FDA and a clinician new to DEMR will not be as experienced as experts that have had a hand in testing and developing it. Ideally, the clinician, novice or not, would be provided with feedback from an expert.
There has been a great deal of work on DEMR, but there have been few schemes described for automatically segmenting it. In one known technique, a region of viable myocardium in a DEMR image is manually isolated, its average intensity found, and pixels with intensities two standard deviations above are labeled as non-viable. Previous MRI techniques have either used cardiac morphology and function alone to determine tissue viability, or have examined the relationship between the amount of non-viable tissue and resulting cardiac function.
As mentioned above, DEMR is prone to false negatives. And, traditional techniques for segmentation such as region growing or thresholding of DEMR yield inadequate results.